1. Field of the Invention
The present invention relates to a mobile satellite communication system, and more particularly to a transmission reservation in a mobile satellite communication system which uses a randomly access system and a reserving system.
2. Description of the Related Art
A communication system which utilizes a randomly access control system and a reserving system described below is well known as a communication system in a mobile satellite communication system for performing a communication between a mobile station and a base station apparatus 1 through a satellite.
At first, a mobile station trying to perform a communication transmits a short burst transmission having a long guard bit pattern to a base station apparatus 1 through an upward control channel. Usually, a plurality of mobile stations are assigned to the upward control channel. Thus, there may be a possibility that competition relation for the use of the upward control channel occurs between the respective mobile stations. For the purpose of arbitration the competition relation, a collision control bit pattern is transmitted from the base station apparatus 1 to each mobile station through a downward control channel. The collision control bit pattern includes an I/B (Idle/Busy) flag indicating whether or not the transmission can be performed by use of the upward control channel. Each mobile station always monitors the collision control bit pattern transmitted through this downward control channel. Only when the I/B flag of the collision control bit pattern is set in an xe2x80x9cIxe2x80x9d state of transmission permission, the short burst transmission is transmitted, and further a clocking operation of a timer is started.
Upon reception of this short burst transmission, the base station apparatus 1 sends a partial echo signal (hereafter, to be referred to as a PE signal) back to the mobile station. The transmission of the short burst transmission from the mobile station to the base station apparatus 1 is ended in this way. If the transmission data from the mobile station to the base station apparatus 1 can be accommodated in the short burst transmission, the transmission from the mobile station to the base station apparatus 1 is ended at this time.
When the I/B flag of the collision control bit pattern is set to the xe2x80x9cIxe2x80x9d state of transmission permission, the short burst transmission may be often transmitted by a plurality of mobile stations at the same time. In this case, the receiving operation is performed in the base station apparatus 1, in a condition that the short burst transmissions from the plurality of mobile stations collide with each other. As a result, an error is detected in a CRC check operation to the received short burst transmissions. For this reason, the base station apparatus 1 treats the short burst transmissions from the respective mobile stations as non-reception.
On the other hand, the mobile station carries out the clocking operation of the timer and after the short burst transmission. The mobile station determines that the short burst transmission fails, when the timer is set to the state of time-out without any reception of the PE signal from the base station apparatus 1 after the short burst transmission. Then, the mobile station again tries the short burst transmission.
The above-mentioned description gives the communication between the mobile station and the base station apparatus 1 when the data transmitted by the mobile station can be accommodated in the single short burst transmission.
On the contrary, there may be a case in which the transmission data from the mobile station to the base station apparatus 1 is long so that the transmission data cannot be accommodated in the single short burst transmission. In this case, the base station apparatus 1 makes a reservation for use of the upward control channel by the mobile station having transmitted the short burst transmission for transmission of subsequent data. Then, the base station apparatus 1 transmits a collision control bit pattern including the PE signal indicative of the specification data identifying the above mobile station to the mobile station through the downward control channel. Upon reception of this PE signal, the mobile station transmits a long burst transmission including the subsequent data through the upward control channel.
The above-mentioned description gives the communication from the mobile station to the base station apparatus 1 by use of the reserving system.
FIGS. 1A-1 and 1A-2, to 1G-1 to 1G-2 are time charts showing an actual example of a procedure of reserving the upward control channel in the mobile satellite communication system as mentioned above. In this mobile satellite communication system, the communication between the base station apparatus 1 and the mobile station is performed in accordance with a time divisional process. Each mobile station can carry out the transmission to the base station apparatus 1 through the upward control channel, by use of units obtained by dividing a time axis into predetermined time lengths. Moreover, the base station apparatus 1 can use each unit to thereby carry out the transmission through the downward control channel to each mobile station.
In the example shown in FIGS. 1A-1 and 1A-2, to 1G-1 to 1G-2, reservations of the upward control channel are made between the base station apparatus 1 and mobile stations A and B. In this example, it is supposed that a transmission delay between each mobile station and the base station apparatus 1 is four frames (one frame is a time length corresponding to one unit).
In FIGS. 1A-1 and 1A-2, to 1G-1 to 1G-2, when receiving the collision control bit pattern through the downward control channel at a unit No. 0, the mobile station A requesting the transmission to the base station apparatus 1 determines whether or not an I/B flag of this collision control bit pattern indicates xe2x80x9cIxe2x80x9d or xe2x80x9cBxe2x80x9d. In this example, the I/B flag is set to the xe2x80x9cIxe2x80x9d state of transmission permission. Accordingly, the mobile station A transmits a short burst transmission 1/6A at a unit NO. 2. After this short burst transmission, the mobile station A starts a clocking operation of a radio channel connection failure timer.
The base station apparatus 1 receives this short burst transmission 1/6A from the mobile station A at a unit NO. 6 delayed from the unit NO. 2 by the four frames. The base station apparatus 1 detects existence of long burst transmissions following the short burst transmission from the data of this received short burst transmission 1/6A. At this time, the base station apparatus 1 transmits PE signals PEA1, to PEA5 in which the number thereof corresponds to this subsequent long burst transmissions. Further, the base station apparatus 1 carries out transmission reservation management of the long burst transmissions from the mobile station A.
On the other hand, the mobile station A carries out the clocking operation of the radio channel connection failure timer, after the short burst transmission, as mentioned above. If the mobile station A does not receive any PE signal designating the mobile station A until the time-out of this radio channel connection failure timer, the mobile station A recognizes it as xe2x80x9cradio channel connection failurexe2x80x9d. Then, the mobile station A voluntarily ends a reception waiting process of the PE signal, and then carries out a retransmission process of again transmitting the short burst transmission.
However, in this example, the PE signals PEA1 to PEA5 are transmitted from the base station apparatus 1 to the mobile station A through the downward control channel. The first PE signal PEA1, is received by the mobile station A at a unit NO. 12 prior to the time-out of the timer. Accordingly, the mobile station A stops the radio channel connection failure timer, and then transmits a long burst transmission 2/6A at a unit NO. 14.
In this example, since continuously receiving the PE signals PEA1, to PEA5, designating the mobile station A, the mobile station A transmits long burst transmissions 3/6A to 6/6a following the long burst transmission 2/6A.
On the other hand, the base station apparatus 1 waits for a long burst transmission from the mobile station A at specified units having unit NOs. 18 to 22, based on the reservation management of the upward control channel.
The transmission delay between the base station apparatus 1 and the mobile station is large in this system. For this reason, after the mobile station transmits the short burst transmission, it takes a long time for the base station apparatus 1 to carry out the transmission reservation of the long burst transmission in accordance with the PE signal. Meanwhile, the I/B flag of the collision control bit pattern continues to be transmitted from the base station apparatus 1 in the xe2x80x9cIxe2x80x9d state. Therefore, in the example shown in FIGS. 1A-1 and 1A-2, to 1G-1 to 1G-2, the other mobile station B transmits a short burst transmission 1/5B within the period from the reception of the short burst transmission by the base station apparatus 1 to completion of the transmission reservation of the long burst transmissions. In this case, after receiving the short burst transmission 1/5B from the mobile station B, the base station apparatus 1 carries out the generation of a PE signal and the transmission reservation, similarly to the case of the mobile station A. Then, after transmitting the PE signals designating the mobile station A for the five units, the base station apparatus 1 transmits PE signals designating the mobile station B. In this case, the base station apparatus 1 determines the units, at which the PE signals are transmitted, in such a way that the long burst transmissions from the mobile station B are received after the reception of the long burst transmissions from the mobile station A.
By the way, the above-mentioned conventional mobile satellite communication system has problems described below. That is, there is a case where the number of mobile stations to be managed by the base station apparatus 1 increases, or the number of long burst transmissions transmitted form the mobile stations is large. In this case, the base station apparatus 1 receives the short burst transmissions from the plurality of mobile stations successively for a short time. For this reason, the transmission of the PE signal from the base station apparatus 1 to a mobile station later transmitting a short burst transmission is delayed. Accordingly, the mobile station recognizes the situation as radio channel connection failure and then proceeds to the retransmission process. As a result, the traffic on the upward control channel is increased more and more.
If the mobile station transmitting the short burst transmission to the base station apparatus 1 proceeds to the retransmission process because of the above reason, a transmission disable frame is generated in which all the mobile stations can not use the upward control channel. As a result, the transmission ability of the upward control channel is reduced.
This problem will be specifically described below with reference to FIGS. 1A-1 and 1A-2, to 1G-1 to 1G-2. As shown in FIGS. 1A-1 and 1A-2, to 1G-1 to 1G-2, it is supposed that the base station apparatus 1 receives the short burst transmission 1/6A from the mobile station A at a unit NO. 6 and then receives the short burst transmission 1/5B from the mobile station B at a unit NO. 7. The generation of the PE signal and the transmission reservation by the base station apparatus 1 are carried out in the order of reception of the short burst transmissions. Therefore, in the example shown in FIGS. 1A-1 and 1A-2, to 1G-1 to 1G-2, the transmission of the PE signal to the mobile station B is carried out after the completion of the transmission of the PE signal to the mobile station A.
In this case, if the setting time of the radio channel connection failure timer of the mobile station B is 10 frames, the radio channel connection failure timer of the mobile station B is set to the time-out state before a unit NO. 17 at which the PE signal from the base station apparatus 1 arrives. Thus, the mobile station B has already ended the reception waiting process of the PE signal. However, when receiving the short burst transmission from the mobile station B, the base station apparatus 1 can not predict that the mobile station B proceeds to the retransmission process, as mentioned above. Therefore, the base station apparatus 1 already carries out the transmission reservation for the long burst transmission from the mobile station B, and already transmits the PE signals PEB1 to PEB4 to the mobile station B.
If the transmission reservation for the mobile station B is carried out and then the PE signals PEB1 to PEB4 are transmitted as mentioned above, another mobile stations having transmission requests and the mobile station B itself, which is in the state of waiting for the transmission of the short burst transmission because of the retransmission process, are not allowed to carry out the transmission. Therefore, this results in the generation of a transmission disable frame 32.
In addition to the above conventional example, a reservation system is described in Japanese Examined Patent Application (JP-B-Showa 59-28301). In this reference, a line is divided into time slots and a plurality of stations uses the line in common in a time divisional multiplexing manner. One station adds the highest priority indication to a reservation burst to indicate that the station has the highest priority of slot allocation, and sends it. When receiving it, each of the other stations calculates its station priority in accordance with a predetermined rule to perform slot allocation for its station independently. Thus, the highest priority indication is notified to the other station in accordance with a predetermined rule.
Also, a multi-connection communication system is described in Japanese Laid Open Patent Application (JP-A-Heisei 1-168126). In this reference, communication between a plurality of child stations is controlled by a parent station through a satellite line. Each of the child stations is composed of a buffer, an information adding section, a transmitting section, and a receiving section. The buffer stores packet data to be transmitted. The information adding section reads out the packet data from the buffer after a time elapses, and adds information to the packet data. The transmitting section transmits the packet data with the information added. The receiving section receives data from the parent station and the other child stations. The parent station is composed of a receiving station, a collision detecting section, a slot managing section, and a transmitting section. The receiving section the packet data transmitted from the child station. The collision detecting section detects collision of the packet data. The slot managing section reads out the information from the buffer to make reservation of the packet data. The transmitting section transmits the information of the reservation and the detecting result of the collision detection section to the child station.
Also, a satellite communication system is described in Japanese Laid Open Patent Application (JP-A-Heisei 2-298127). In this reference, a plurality of ground stations commonly uses a radio frequency through a communication satellite in response to connection request signals from terminals. The plurality of ground stations are allocated in advance with individual station numbers, respectively. A transmission and reception unit of each ground station is composed of an adding section, a control section, a detecting section, and a determining section. The adding section adds the station number to a transmission signal. The control section control starts and stop of transmission in response to a connection request signal inputted from the terminal. The detecting section detects the station number contained in a reception signal which is transmitted from the ground station and returned to the ground station by the communication satellite. The determining section determines whether its station number is correctly detected from the reception signal before a predetermined protection time is elapses after the transmission is started in response to the connection request signal. When its station number is not correctly detected from the reception signal, an alarm signal is outputted to the terminal to indicate abnormality of a transmission line while the connection request signal is inputted from the terminal.
Also, a satellite communication system is described in Japanese Examined Patent Application (JP-B-Heisei 3-62329). In this reference, a plurality of peripheral stations access a center station through a satellite and a common channel. Each of the plurality of peripheral stations determines whether a transmission data of its station is a shorter or longer than a predetermined number of slots. Also the peripheral station makes reservation for transmission time slots to transmit the long data, and transmits the short data using a time slot which is not reserved. The center station is composed of a traffic detecting section. The traffic detecting section detects a total traffic amount from the plurality of peripheral stations and outputs an instruction to the peripheral stations to shorten a time slots length when the detected total traffic amount is larger than a predetermined value. Each peripheral station is composed of a slot length setting section for changing and setting the time slot length in accordance with the instruction.
Also, a radio reservation system is described in Japanese Laid Open Patent Application (JP-A-Heisei 4-256231). In this reference, an information control section 32 of a mobile station 21 makes reservation registration of a line to a reservation information storing section 35 when the communication channels are all in a traffic jam state. The information control section 32 automatically originates a call when an empty portion is detected in the line. Further, the information control section 32 stores a new identifier in an area identifier storing section 36 when a station area is changed. An automatic call originating operation is performed to a new station. The information control section 32 makes reservation registration of a line to a reservation information storing section 35 when the communication channels are all in a traffic jam state, and automatically originates a call when an empty portion is detected in the line.
The present invention is accomplished to solve the above-mentioned problems. Therefore, an object of the present invention is to provide a method and system for making transmission reservation in a mobile satellite communication system which can prevent a mobile station from uselessly proceeding to retransmission process of a short burst transmission even if a traffic is increased.
In order to achieve an aspect of the present invention, a mobile satellite communication system includes a plurality of mobile stations, and a base station apparatus connected to the plurality of mobile stations through a communication satellite. The base station apparatus performs transmission to the plurality of mobile stations using a single downward control channel and reception from the plurality of mobile stations using a single upward control channel. Each of the plurality of mobile stations performs a short burst transmission requesting long burst transmissions. The base station apparatus includes a partial echo (PE) signal buffer for storing an identifier of each of mobile stations requesting long burst transmissions, the identifier being contained in a PE signal, a PE signal transmission counter for storing a number of long burst transmissions as a transmission count for each of the requesting mobile stations, and a transmission reservation count storing buffer for storing a number of the requesting mobile stations as a transmission reservation count, the transmission reservation count being updated in response to the requesting short burst transmission. A control unit transmits a collision control signal including the PE signal based on a maximum allowable transmission reservation count and the transmission reservation count in response to the requesting short burst transmission.
The mobile satellite communication system may further include a maximum allowable transmission reservation count buffer for storing the maximum allowable transmission reservation count. In this case, the control unit reads out the maximum allowable transmission reservation count from the maximum allowable transmission reservation count buffer and performs the transmission of collision control signal, based on the read maximum allowable transmission reservation count and the transmission reservation count.
When the collision control signal is transmitted to one of the requesting mobile stations, the control unit decrements the transmission count for the one requesting mobile station by xe2x80x9c1xe2x80x9d. Also, the control unit decrements the transmission reservation count by xe2x80x9c1xe2x80x9d when the collision control signal is repeatedly transmitted to one of the requesting mobile stations for the transmission count.
The control unit transmits the collision control signal including the PE signal to one of the requesting mobile stations determined based on the maximum allowable transmission reservation count and the transmission reservation count in response to the requesting short burst transmission.
In this case, when the transmission reservation count is smaller than the maximum allowable transmission reservation count, the control unit transmits the collision control signal including the PE signal to one of the requesting mobile stations continuously for the transmission count. Also, when the transmission reservation count is equal to or larger than the maximum allowable transmission reservation count, the control unit circularly transmits the collision control signals to the requesting mobile stations one by one for every transmission time unit.
Each of the plurality of mobile stations may include a timer which is set to a time-out state in a predetermined number of transmission time units. In this case, an allowable number of the requesting mobile stations is determined based on a communication delay time between the base station apparatus and each of the plurality of mobile stations and the predetermined number of transmission time units of the timer.
In order to achieve another aspect of the present invention, a method of transmitting a data from each of a plurality of mobile stations to a base station apparatus through a communication satellite in a mobile satellite communication system, wherein transmission from the base station apparatus to the plurality of mobile stations is performed using a single downward control channel and transmission from the plurality of mobile stations to the base station apparatus is performed using a single upward control channel, comprising the steps of:
performs a short burst transmission requesting long burst transmissions from each of requesting ones of the plurality of mobile stations to the base station apparatus;
updating a transmission reservation count in response to the requesting short burst transmission;
transmitting a collision control signal including a partial echo (PE) signal from the base station apparatus to the requesting mobile stations for numbers of long burst transmissions as transmission counts based on a maximum allowable transmission reservation count and the transmission reservation count in response to the requesting short burst transmission, the PE signal including an identifier of each of the requesting mobile stations; and
transmitting the data from each of the requesting mobile stations to the base station apparatus.